1. Field of Invention
The present invention relates to a porous web, useful as a filter element and to a filter device comprising said porous web, particularly for filtration of biological fluids.
A specific embodiment of the invention relates to filter devices for filtration of blood and blood components, although the invention is not intended to be limited to such a specific embodiment.
2. Description of the Prior Art
Porous elements comprising a plurality of adjacent melt-blown webs or layers are conventionally used for filtration of blood and blood components, particularly for the removal of leukocytes therefrom.
In the melt-blowing process, which has been successfully used in the production of non-woven fabrics since nearly fifty years, a thermoplastic fibre forming polymer is extruded through a linear die containing several hundred small orifices; streams of hot air, exiting from the left and right side of the die, rapidly attenuate the extruded polymer streams to form extremely fine diameter fibres.
The attenuated fibres subsequently get blown by high velocity air on a collector screen, thus forming a fine self-bonded non-woven melt-blown web in a single integrated step. Melt-blown webs, wherein the fibre networks result from the stochastic spatial deposition of fibres, have been generally considered to be constituted by randomly distributed fibres.
Only in recent years, the multi-hole melt-blowing process has been partially understood from a physical point of view, with the help of imaging techniques and powerful mathematical/physical modelling methods, such as for example Computational Fluid Dynamics (CFD).
Actually, the melt-blown webs do have orientation; the orientation depends from material rheology, melt-blowing equipment design (especially from suction, belt and die block design), equipment set-up and greatly from the process conditions, particularly the aerodynamic and thermal conditions.
In the commercial melt-blowing process, there two well-identified directions that are called Machine Direction (MD)—the time forward direction, along the length of the roll in production and Cross Direction (CD)—equivalent to the melt-blowing line width, also called “roll width” or “roll height” direction.
Even if it is difficult to describe a priori web anisotropy given all the variables of the melt-blowing process that might influence the final result, it is relatively easy to get an estimation of web anisotropy once the non-woven web is produced.
A very simple tensile strength test—performed according to one of the international standards (for example the EDANA 20.2-89 standard) on a dynamometer—gives a clear indication of the web orientation by means of two parameters: the breaking load (or breaking strength) of the web and the elongation at maximum load (elongation at break), measured on a given material in both MD and CD. The ratio of MD/CD values of said tensile properties is commonly understood as an indication of fibre orientation.
More recent studies on the microscopic structure of webs confirmed that the MD/CD ratio of a different parameter, the fibre cross-sectional area, is also a useful and more detailed method for describing web anisotropy (cf. “Influence of Processing Conditions on Melt-Blown Web Structures: part 1—DCD”, by Randall R. Bresee, Department of Materials Science & Engineering, The University of Tennessee, Knoxville, Tennessee and Uzair A. Qureshi, Jentex Corporation, Budford, Ga., INJ, issue Spring 2004).
Even though the breaking load of a given web can be modified by means, for example, of hot calendering (i.e. modifying the intrinsic thickness of the material and the number of fibreto-fibre contacts), the MD and CD remain clearly distinguishable through different elongation at break.
Also the Fraunhofer Diffraction method has been successfully proposed as a technique to assess the Fibre Orientation Distribution Function (ODF) (cf. “Study of Melt-blown Structures Formed by Robotic and Melt-blowing Integrated System: Impact of Process Parameters on Fibre Orientation” by Raoul Farer, et al., College of Textiles, North Caroline State University, Raileigh, USA, INJ, issue Winter 2002).
The anisotropy of the fibres in the web is directly related to the pore structure (“pore shape”) of the web. Recent studies also proved that the wetting properties of a non-woven fabric are markedly influenced by web structure anisotropy.